The invention is in the field of information storage and retrieval systems, and relates to memories, such as computer memories. The invention is specifically directed to a memory which is very small in size but can store a very large amount of information.
There is an ever growing need for efficiently storing and retrieving large amounts of information, and there are many different storage systems which try to optimize, to varying degrees, factors such as storage capacity, access time, size, cost and the like. The problem is particularly severe where it is desired to store a very large amount of information and to have any portion of its accessible in a minimum time, e.g. the problem of storing the Chemical Abstracts or the problem of storing the inventory records of all the spare parts maintained by the Navy throughout the world. The memories typically used at present, such as magnetic core and semiconductor and similar memories, are satisfactory for many uses, but do not have sufficiently high storage density for many other uses. Research efforts have therefore been directed to alternate storage techniques, such as optical storage. An example of such techniques is the Unicon 690 Laser Store for the Iliac IV computer installation, which has a storage density of about 2.7.times. 10.sup.7 bits per square inch. Even with optical memories, however, the diffraction limit of the imaging lens imposes a theoretical limit of about 10.sup.8 bits per square inch, and there is a major technical difficulty in providing high beam deflection to cover a large field. There are, additionally, optical storage memories which employ light sensitive color centers (F-centers, M-centers and M.sub.A -centers) in crystals-- as discussed, for example, in U.S. Pat. Nos. 3,440,642, 3,673,578 and 3,720,926, but since again light beams are involved in such memories, the same theoretical limit of about 10.sup.8 bits per square inch is present. A somewhat different approach is taken when using electron beam modulation of storage media. For example, U.S. Pat. No. 3,723,978 relates to a technique using an electron beam to locally charge portions of a storage medium which retain the charge for up to fifty hours, while U.S. Pat. No. 3,760,383 relates to a technique for permanent local thermal destruction of selected macroscopic portions of a storage medium by an electron beam. It appears, however, that the storage density of such electron beam systems is no better than with the optical systems discussed above.